Recently, semiconductor laser devices are frequently used as light sources of optical pickups used in optical recording apparatuses or optical reading apparatuses for recording media such as optical disks and magneto-optical recording disks. The light sources of optical pickups are applied to a variety of fields of recorders, PCs (personal computers) and on-vehicle apparatuses, and the optical disk market has been continuously expanding. In particular, there is a large demand for on-vehicle apparatuses typified by car navigation systems, and the demand for an optical pickup usable for reproducing CDs (compact disks) and DVDs (digital video disks), namely, all kinds of disks, is increasing.
An on-vehicle optical pickup is strongly required of (1) compactness of the optical pickup, (2) suppression of signal degradation (low noise) and (3) a wide range of operation temperature assurance for operating in a temperature range from a low temperature to a high temperature.
First, for attaining (1) compactness of the optical pickup, it is effective to simplify the device by reducing the number of optical components, and as one method useful for this purpose, a monolithic semiconductor laser in which a red semiconductor laser of a 650 nm band for a DVD and an infrared semiconductor laser of a 780 nm band for a CD are integrated on one semiconductor substrate is realized. Thus, a plurality of semiconductor lasers are aggregated on one substrate so as to reduce lead time and cost as well as optical components such as a collimator lens and a beam splitter can be shared by the red semiconductor laser and the infrared semiconductor laser, and hence this method is useful for attaining the compactness.
Next, with respect to (2) suppression of signal degradation (low noise), a factor for causing noise will be first described. When a semiconductor laser is used as a light source of an optical disk system, laser beams collected on an optical disk are reflected on the surface of the optical disk so as to be fed back to an end face for emitting laser. In this case, the optical disk functions as if it were a complex resonator. An oscillation wavelength of the longitudinal mode depending upon a complex resonator and an oscillation wavelength of the longitudinal mode depending upon a resonator face of the semiconductor laser itself are different because the length of the optical path is different between these resonators. Furthermore, the effective end face reflectance caused by the complex resonator effect is varied depending upon the length of the optical path. Therefore, in a given operation state, the value of an oscillation threshold current of the longitudinal mode depending upon the complex resonator is smaller than the value of an oscillation threshold current of the longitudinal mode depending upon the laser itself, and as a result, the oscillation mode may be exchanged. At this point, mode competition is caused in the longitudinal mode by feedback light reflected on the optical disk, so as to make the optical output unstable, resulting in causing noise. Such noise is designated as external feedback noise. When the amplitude, calculated as relative intensity noise (RIN), of this noise exceeds −120 dB/Hz, there arises a practical problem.
In order to reduce the external feedback noise in a semiconductor laser, it is effective to operate the semiconductor laser in multi longitudinal mode. The operation in the multi longitudinal mode means a laser oscillation state in which the wavelength for causing laser oscillation (oscillation wavelength) is not a single wavelength but includes a plurality of oscillation wavelengths. When the semiconductor laser is operated in the multi longitudinal mode, the mode competition is minimally caused, and excessive noise can be avoided, and therefore, a low noise characteristic minimally affected by feedback light reflected on an optical disk can be realized.
In order to cause multimode oscillation, a semiconductor laser is operated in the form of pulse. In order to operate the semiconductor laser in the form of pulse, the laser is pulse-driven by using a high-frequency superimposed circuit. In this case, however, an additional driving circuit is necessary, which increases the number of components and hence is disadvantageous for the compactness or the cost of the optical pickup. Furthermore, a recent car includes a large number of accessories using high frequencies (such as an ETC (electronic toll collection) system) apart from a high frequency superimposed module for rapidly modulating the semiconductor laser device. Therefore, it is apprehended that a problem such as malfunction of equipment derived from frequency resonance between these equipment may be caused. Accordingly, this method for rapidly modulating the semiconductor laser device cannot be the best method.
In order to drive a laser in the form of pulse without using a high frequency superimposed circuit, it is effective to use a self oscillation phenomenon of the semiconductor laser. In a self oscillation type laser, it is necessary to form a saturable absorber whose absorption region for a laser beam formed in a waveguide is excited by the laser beam itself so as to reduce the light absorption and to ultimately become transparent (absorption saturated). When the absorber becomes transparent, the loss of the waveguide is reduced and the optical output is abruptly increased. When the optical output is increased, the number of carriers consumed in an active layer on stimulated emission is increased, and hence the carriers are abruptly lost, and ultimately the number of carriers becomes so small that the laser oscillation is halted. Therefore, the optical output of the self oscillation type laser is operated in the form of pulse with respect to time even when it is operated with a DC bias, and thus, the multi longitudinal mode oscillation is attained. Moreover, in the semiconductor laser operated in the self oscillation manner, the carrier density in the active layer is oscillated with time, and hence, the refractive index of the active layer is varied with time. When the refractive index of the active layer is varied, the emission wavelength is also varied. Therefore, the line widths of respective oscillation spectra are increased, so that interference with the feedback light reflected on an optical disk can be reduced. In this manner, for (2) suppression of signal degradation (low noise), a self oscillation type laser is significant as a compact light source with small noise in which excessive noise can be reduced without using a high frequency superimposed circuit.
Next, for (3) a wide range of operation temperature assurance, it is necessary to improve the temperature characteristic of the semiconductor laser device itself. As one method for this purpose, a method described in Patent Document 1 is known. Patent Document 1 describes that the temperature characteristic of a semiconductor laser device is improved by setting the impurity concentration in a cladding layer to 8×1017 cm−3 or more.
Patent Document 1: Japanese Patent Application Laid Open Gazette No. 7-38200